Top coating paint for automobiles generally contains, as a film forming binder, a combination of a hydroxyl group containing polymer and a melamine curing agent. The cured film obtained from the melamine curing system, however, has poor acid resistance and is damaged by acid rain which has recently become one of serious problems. The damage of the film provides poor appearance.
In order to overcome the above mentioned defects, the present inventors have proposed novel coating compositions without employing the melamine curing agent in Japanese Kokai Publications 45577/1990, 287650/1991. The similar coating compositions are also disclosed in U.S. Pat. No. Zopf et al. 2,977,334, Zimmerman et al. 3,399,109 and Marion et al. 3,528,935. The proposed coating composition cures by means of the reaction of acid with epoxy and therefore has good acid resistance and sufficient weather resistance for an automobile top coating application.
However, these coatings have a problem that they are easily brushed by a car washing machine. If degree of crosslinking is raised in order to impart good mar resistance at the mechanical car washing in this curing system, the resulting film has poor elongation and is stiff and brittle. The stiff and brittle film has poor mar resistance and is easily damaged by scratch. It is therefore difficult to use the proposed coating compositions providing merely a high degree of crosslinking as a top coating paint for the automobiles, because such a composition provide poor mar resistance at the mechanical car washing. It is desired to develop top coating paint which has not only excellent acid resistance but also high mar resistance.